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Measureing AC line voltage.

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Measureing AC line voltage.

Postby mnollid » Wed Mar 30, 2005 1:01 am

I'm useing my ADC-200 to mesure 120 VAC, but I'm actually getting a reading of 160 VAC on my PC Scope, and I'm not sure why?

I've used the signal generater and calibrated my probes, but still reading 160 VAC on a 120 VAC circuit.

Please help.

Thank you.

Postby Sarah » Wed Mar 30, 2005 9:07 am


Thank you for your post.

Where is the reading displaying from? Is it the meter view, or is it by using the Settings | Measurements option? I would recommend trying both to see if one is giving the wrong reading or if they both display the same.

Let me know how you get on.

Best Regards

Measureing AC Line Voltage.

Postby mnollid » Wed Mar 30, 2005 6:06 pm

Thank you for your help.

I should have given more information, sorry about that.

I'm using XP profesional, and pico version 5.12.1, and when looking at the version, I also see an error ADC 200 DRIVER V 4.02, ERROR 13 (H255 SO).

I'm using X10 probes, and taking the readings from the Oscope view. I tried to mesure with the meter view but couldn't get it to work? I've got the settings for probes at X10, and my 60hz trace reading is showing 160 VAC when measureing a 120 VAC line.
I took a 9V battery and switched my probes to X1, and got a perfect 9VDC reading. I switched my probe back to X10 and still got a perfect 9VDC reading. So it seams my probes are calibrated correctly. I also used the signal gen. output fromt he ADC 200 and got correct readings.

But when trying to measure the 120 VAC line, I read 160 VAC. I haven't checked the voltage at the probe with a hand held meter yet. I was going to try this using the pico meter view, but couldn't get it to work.

Postby Sarah » Thu Mar 31, 2005 8:56 am


Thank you for your post and the extra information.

Can you tell me exactly what it says in the Help | About dialogue? - All the information that it gives? It seems strange that it appears to give an error code when the unit is working fine.

I would recommend measuring the voltage at source with another meter as in every other way the Picoscope seems to be working fine. Let me know how you get on with this.

Best Regards

Measuring AC Line Voltage.

Postby mnollid » Wed Apr 06, 2005 1:23 am

Copyright 1995-2005 Pico Technology Ltd.

Release 5.12.1 - 32 bit.

ADC200 Driver V4.02
Error 13 [H255 SO]

ADC200.sys V6.0

Thanks for your help.[/list]

Measuring AC Line Voltage

Postby mnollid » Wed Apr 06, 2005 2:45 am

Now I'm getting confused.

I have tested the 10X probe by connecting to a 120VAC source and measuring the AC voltage at the BNC connector with a calibrated Fluke 83III multimeter. When I ground the probe I'm reading 60VAC at the BNC, 1/2 of the 120VAC? This is where I'm getting confused. Shouldn't I be seeing 1/10 of this voltage at the BNC side of the 10X probe? 1/10 X 120 = 12VAC.

The max input voltage to the ADC200 is 20 V right? Is this DC or AC?

When I connect the probe to the 120VAC source grounded using the 1X setting, I get the full 120VAC.

When I read a 9VDC batter with the 10X probe I read half the voltage at the BNC (4.5 VDC). When switching to 1X I read the full 9 VDC.

The PicoSocpe reads the DC voltage and displays it to the scope view correctly. The 5V Signal Gen. also reads correctly.

But when trying to read the 120VAC, the Scope view reads 160VAC?

I'm so confused on what is going on here.

Postby Sarah » Wed Apr 06, 2005 11:46 am


Thank you for your post.

There are a couple of things here to consider. Firstly, a voltage of 120VAC will give a peak voltage on the sine wave of around 170V.

The second more inportant thing is where you take the reading with reference to. The 120VAC will be between the Live and the Neutral connections. The neutral is not the same as the Earth as there may be a voltage difference. Therefore if your readings are referenced to Earth (as they normally are with a single ended probe) then they could be showing higher.

What you need to do is use an isolating differential probe. It needs to be isolating due to the size of the voltage, and it needs to be differential so that you can measure between Live and Neutral.

The 20V input to our unit is a DC limit. This should not be exceeded.

I am not sure why your probe is showing the readings that it is, it may be that it is faulty.

Hope this helps

Best Regards

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